1. Field of the Invention
The present invention relates to an aqueous drilling fluid composition and method of using same in drilling oil and gas wells and the like. More particularly, the present invention concerns incorporation of at least water dispersible terpolymers or salts thereof prepared in part from a monomer which is an unsaturated polybasic acid as a fluid loss control additive in a drilling fluid used in drilling wells in subterranean formations.
2. Description of Prior Art
Rotary systems are generally used in drilling oil and gas wells. These systems depend upon rotation of a drill pipe, to the bottom of which is attached to a drill bit. The bit cuts into the formation causing the cuttings to accumulate as drilling continues. A drilling fluid is used which lubricates the drill bit and carries cuttings to the surface for removal. This allows the bit to continue functioning and the bottom hole to be clean and free of cuttings. The drilling fluid also is the source of pressure which is exerted against the formation. Even in other drilling systems, drilling fluids still are needed to remove bore hole cuttings and to otherwise perform functions related to drilling fluids.
Oil and gas producing formations are generally porous layers having varying degrees of permeability to the flow of fluids such as oil, water or gas. When drilling through such porous formations, it is essential to employ drilling fluids having characteristics such that excessive amounts of the drilling fluid do not penetrate the porous formation. Drilling fluids have a tendency to penetrate the formation because the pressure in the bore hole is greater than the pressure opposing it which comes from the formation. Should excess penetration occur, there is loss of drilling fluid into the formation resulting in loss of pressure, and the inability to drill and remove cuttings.
There are typically three types of drilling fluids: (1) water-base (2) hydrocarbon or oil base emulsion and (3) air, mist or foam. Combinations are also used. With each of these types of drilling fluids (or muds) there are numerous systems or variations. The type of drilling fluids selected depends on geological and temperature environment and other performance and economic criteria.
Water base drilling fluids (or muds) usually contain water as the main component, a pH control agent (NaOH, KOH, etc.) a weighting agent (barite, hematite, calcium carbonate, etc.) a viscosifier (bentonite clay, attapulgite clay, high molecular polyacrylate/acrylamide, etc.) and a dispersant (lignite, lignosulphonate, modified tannin, low molecular weight polyacrylate, etc.).
Other additives can be used to impart desired chemical or physical properties to these water base fluids to enhance positive interaction to the geological formation. For example, NaCl salt is used to saturate the aqueous phase of the mud if a salt formation is to be diluted. This has the positive effect of decreased hole wash out and increased well life due to good cement bonding.
Lime, calcium hydroxide, can be added to a drilling fluid to obtain certain properties in limestone or hydratable shale formations. Lime drilling fluid (or mud) systems are characterized by having a very high pH (11-12.5) and a saturated aqueous phase with respect to calcium hydroxide with an excess of undissolved calcium hydroxide.
The lime drilling fluid systems may or may not incorporate potassium hydroxide as an inhibiting additive. Calcium is incorporated in these lime drilling fluids (or muds) to chemically obtain a desired rheological yield point, gel strengths and shale stability. Potassium can be added to supplement shale stabilization and is believed to control and stabilize clays not geometrically suitable for the large hydrated calcium ion. Calcium is believed to base exchange and convert sodium hydrated shales to divalent, calcium shales. Clay platelets are dehydrated, thereby increasing a high solids content with minimum viscosity and gel strengths to the mud.
Gypsum, calcium sulfate, can also be added to drilling fluids to obtain certain properties in anhydrite and hydratable shale formations. These "gyp" muds are similar to lime base muds with respect to their soluble calcium shale stabilization mechanisms. However, "gyp" muds differ from "lime" muds in that "gyp" muds have a lower pH (9-10.5) and higher soluble calcium content due to these pH changes.
These "lime" and "gyp" muds are significantly different than typical freshwater drilling fluids (or muds) contaminated with calcium. In freshwater drilling fluids (or muds), typical contaminates are drilled into, or contact the fluid as the result of the drilling operation and are not purposefully added to or incorporated into the fluid system. These freshwater muds will have a lower calcium concentration in solution. U.S. Pat. No. 2,802,783 to W. J. Weiss et al. describes a lime drilling fluid which has been used successfully for drilling heavy shale formations in the Gulf Coast Region. Holt et al., "The Use of Potassium-Lime Drilling Fluid Systems in the Navarin Basin Drilling", Society of Petroleum Engineers (SPE) Paper No. 14755, presented at the IADC SPE Conference, Dallas, Tex., Feb. 10-12, 1986; and Fleming, "Moderate pH, Potassium, Polymer-Treated Mud Reduces Washout", SPE Paper No. 14758, presented at the IADC SPE Conference, Dallas, Tex., Feb. 10-12, 1986, each describe lime mud formulations which have been employed successfully in oil and gas exploration.
Various materials have been added to "lime" and "gyp" muds to control fluid loss and have included materials such as pre-gelatinized starch; sodium carboxymethyl cellulose; copolymers of (1) a (meth)acrylamido alkyl sulfonic acid, including 2-acrylamido-2-methyl-propane sulfonic acid (which is sold under the trademark "AMPS" by Lubrizol Corporation), and (2) a (meth)acrylamide or N-alkyl (meth)acrylamide or cross-linked copolymers thereof (see, U.S. Pat. No. 4,293,427 to Lucas et al.); polymers of AMPS and N-vinylamides (see, U.S. Pat. No. 4,309,523 to Engelhard et al.); polyacrylates (see, e.g., U.S. Pat. No. 2,718,497 to Oldham et al.); and lignites.
However, these materials have not been satisfactory. For example, polycrylate polymers lose their effectiveness in the presence of calcium ions. Where polyacrylates are added to fresh water muds, fluid loss can be reasonably controlled. However, where soluble calcium is present in the muds, particularly under conditions of high temperature and pressure, polyacrylates are not as effective in controlling fluid loss. Other known copolymers and terpolymers although effective in fluid loss control, cause severe increases in fluid rheology under high temperature environments.